fix(deps-dev): add @babel/plugin-proposal-private-property-in-object

[queengooborg]

This PR adds @babel/plugin-proposal-private-property-in-object as a dev. dependency.

When running yarn dev, the following error message is shown:

One of your dependencies, babel-preset-react-app, is importing the
"@babel/plugin-proposal-private-property-in-object" package without
declaring it in its dependencies. This is currently working because
"@babel/plugin-proposal-private-property-in-object" is already in your
node_modules folder for unrelated reasons, but it may break at any time.

babel-preset-react-app is part of the create-react-app project, which
is not maintianed anymore. It is thus unlikely that this bug will
ever be fixed. Add "@babel/plugin-proposal-private-property-in-object" to
your devDependencies to work around this error. This will make this message
go away.

After installing the dependency as suggested, no other such messages appear to be displayed.